Harry, Meghan, William and Kate joined the Queen at Westminster Abbey this morning as the royals helped kick off a momentous day-long celebration to mark 100 years of the Royal Air Force.

The fab four led a host of dignitaries and politicians, including embattled Prime Minister Theresa May, into the service for the first of several events to take part today, ahead of one of the largest military flypasts in recent memory this afternoon.

Up to 100 aircraft from Battle of Britain-era Spitfires and Lancaster Bombers to Britain's cutting-edge F-35 stealth fighter jets will take to the skies from 12.45pm before thundering over Buckingham Palace 15 minutes later.

Meghan, the Duchess of Sussex, at the service at Westminster Abbey to mark the centenary of the Royal Air Force, in London. Photo / AP
Meghan, the Duchess of Sussex, at the service at Westminster Abbey to mark the centenary of the Royal Air Force, in London. Photo / AP

Thousands are expected to line the streets to catch a glimpse of the iconic flypast, while more than a thousand servicemen and women will also take part in a parade on The Mall.

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William and Kate arrived at the Abbey this morning less than 24 hours after Prince Louis' christening at St James's Palace, attended by Harry, Meghan, Charles, Camilla and the Middletons.

But the Queen and Prince Philip were not there because of a busy week for the 92-year-old monarch, including Trump's three-day visit to the UK from Thursday.

After fulfilling their commitments, Harry and Meghan will fly out Ireland for two days for their first official overseas visit as a married couple.

For today's celebrations, the Duchess of Sussex wore a dress by Dior and hat by royal milliner Stephen Jones, while the Duchess of Cambridge chose to wear a coat by Alexander McQueen and an RAF Dacre Brooch.

Despite still being on maternity leave, after giving birth to Louis only 11 weeks ago, it is understood Kate felt the celebrations were important to attend if she could, having also taken part in Louis' christening yesterday.

It was on April 1, 1918 that the Royal Flying Corps and the Royal Naval Air Service merged to create the RAF - the world's first independent air service.

Following the service at Westminster Abbey, the Queen will present a new Queen's Colour to the RAF along with her son Prince Charles in the forecourt of Buckingham Palace, before she makes a brief speech.

Kate, the Duchess of Cambridge walks ahead of Meghan and Harry. Photo / AP
Kate, the Duchess of Cambridge walks ahead of Meghan and Harry. Photo / AP

The royals will then appear on the balcony of the Queen's London residence to view a flypast of up to 100 RAF aircraft, a parade of more than 1000 personnel, and a feu de joie.

Air-Vice Marshal Gerry Mayhew, the air officer commanding No 1 group and the senior responsible officer for the flypast, said the day is the "centrepiece" of the RAF's 100th year.

"It is the high point for the capital, Her Majesty the Queen and the royal family," he told the Press Association.

"And it is a great opportunity to look back and really celebrate our past, but to look forward is a really important thing for us as we look to the next 100 years."

He described being at the forefront of the flypast as a "great honour", and said he hopes the Queen and royal family will see 'exactly what they can do' for the monarch.

Asked what message a 100-aircraft flypast may send to Britain's adversaries and the world, he said: "It demonstrates we are a competent, professional and capable air force that has a credible, very credible, set of capabilities."

Charles, Camilla, William, Harry and Meghan will also meet veterans, personnel, charity representatives and those from the civil service in a reception after the spectacle.

Just 15 years after the first-ever powered flight undertaken by the Wright brothers, the decision to create the RAF was made after the War Cabinet inquiry criticised the poor organisation of Britain's air forces.

Those recognised as the founders of the RAF include Lieutenant General Sir David Henderson and Lord Hugh Trenchard - the first chief of the air staff.

The flypast is set to feature aircraft including Puma and Chinook helicopters, a Lancaster, Spitfires, Hurricanes, the A400M Atlas, Tornados, Typhoons and the F-35 stealth fighter jets.

Drawing the spectacle to a close will be the Red Arrows, who in traditional aerobatic style will stream red, white and blue smoke down the length of The Mall.

The officer commanding the Red Arrows, Wing Commander Andrew Keith, who took the helm in August, said it is an incredible year to be leading them.

"We will put the red, white and blue on at the end of the flypast, which I think is a fitting way to end it, which is great," he said.

During the flypast, he said he will be flying in the formation in the back seat of the team leader's jet, "getting a few photos".

"It is going to be business as usual for us - and hopefully clocking over that 100th aircraft," he said when asked if they will be doing anything different on the day.